In many applications, microphones with uniformly high sensitivity in directions within an "equatorial" plane and low sensitivity in the direction perpendicular to this plane, that is, along the "polar" axis, are desired. An example is conference telephone, where the microphone should receive the voices of participants seated around a table with uniformly high sensitivity while discriminating against sound reflected from ceiling and table top as well as sound from an overhead loudspeaker.
Such "toroidal" microphones are designed in the prior art using a variety of principles. For example, a transducer comprising two first order gradients, arranged at right angles, whose outputs are added in quadrature phase is disclosed in U.S. Pat. No. 2,539,671 issued Jan. 30, 1951 to H. F. Olson. Another example is a transducer comprising two second order gradients also arranged at right angles, whose outputs are added directly as disclosed by G. M. Sessler et al in a paper which was published in 1971 in the IEEE Transaction on Audio and Electroacoustics, volume AU-19, at page 19. While the former principle yields only a cosine shaped directivity pattern in the polar plane but requires a broadband ninety degree phase shifter, the latter design delivers the more desirable cosine squared characteristic and requires no phase network. In its original implementation, the cosine squared system was difficult to balance acoustically and had a relatively poor signal to noise performance. A new implementation of the second order toroidal microphone is desirable which avoids the shortcomings of the former design.